Magnetic recording media are well known in the arts, and include tapes, discs, drums, and other forms of tablet or continuous loop configurations. These recording media generally comprise a magnetic coating material deposited upon a permanent substrate. The magnetic coating material coating in turn consists of magnetic particles such as ferromagnetic particles comprised of, for example, iron oxide, disposed in a binder, such as an epoxy resin. Often, the binder systems are complex and the ferromagnetic materials may include other ferromagnetic materials in addition to the iron oxide. These coating compositions are disposed upon a permanent substrate by a number of means, such as by dipping or spincoating or spraying and are cured or otherwise hardened to form a permanent part of the structure.
These various magnetic recording media are, of course, utilized in conjunction with magnetic recording heads or transducers to produce the desired reading, read-write, or write properties. Presently, the thickness of a typical media film is on the order of 1 to 2 .mu.m. However, as current technology trends continue, thinner and thinner magnetic recording media are evolving. Within the next few years, it is anticipated that media thicknesses on the order of 0.5 .mu.m will be required. These thinner coatings will be capable of higher density recording with the recording heads being brought closer and closer to physical contact with the recording media. Very often, as in the case of magnetic discs, this results in "crashing" of the head onto the surface of the disc. Not only does this cause wear upon the disc and head surfaces, but the debris caused by such a crash often adheres to the head and affects its aerodynamic properties. If the coating is physically displaced at a point where the head contacts the recording media, the information stored therein may be permanently destroyed.
The prior art, for example, U.S. Pat. No. 3,843,404 and U.S. Pat. No. 3,622,386 recognize the advantage of producing a magnetic recording layer having a high abrasion resistance by incorporating hard, non-magnetic particles within the magnetic binder layer. Ideally, the prior art has taught the use of hard materials such as aluminum oxide (alumina) in controlled quantities and sizes to substantially increase the abrasion resistance and durability of a magnetic coating. Although the prior art further has recognized the current trend to provide for thinner and thinner magnetic recording layers, the prior art has failed, to date, to controllably produce alumina particles of the required small dimension having, ideally, substantially spherical cross-sectional geometries.
The prior art has recognized the advantage of using alumina particles that are substantially equal to the binder media thickness. The bulk of the alumina particles incorporated into the media has been obtained, generally, from ball-mill debris created during the mixing of the binder solution. To this is added additional alumina which has been subjected to ball-milling. However, the size and shape of these particles can be extremely irregular and oftentimes a large alumina particle will be knocked out of the binder during disc burnishing operations, resulting in a void on the disc surface.
As media thickness decreases from the present day approximately 1 to 2 .mu.m thicknesses to the anticipated approximately 0.5 .mu.m thicknesses, small alumina particles together with tighter particle distributions will also be required. Alumina obtained as a ball-milled debris product cannot meet the submicron size and distribution tolerances. Additionally, the quality of supplemental alumina currently used is too large with much too an uneven size distribution to meet these stringent media thickness requirements.
It is thus an object of the present invention to produce a magnetic recording medium having alumina particles of a substantially spherical geometry which are of a size and particle distribution to avoid those problems experienced by the prior art.
It is a further object of the present invention to provide magnetic recording medium substantially less than 1 .mu.m in thickness having, dispersed therein alumina particles substantially no larger than the media thickness and of the size distribution specifically adaptable for use therein.